Toothpaste compositions are well characterised in the literature and many compositions are disclosed in patent specifications and other literature. Toothpaste compositions contain a number of specific components for example abrasive agents, fluoride sources, binders, preservatives, humectants, anti plaque agents, colouring agents, water, flavour and other optional ingredients. Of these components the abrasive agent is required to provide the appropriate cleaning and plaque removal without subjecting the tooth itself to excessive abrasion. Typically a toothpaste composition will contain from about 5% to about 50% preferably up to about 30% by weight of abrasive.
Commonly used abrasives are aluminas, calcium carbonates and calcium phosphates. More recently synthetic silicas have been adopted because of their efficient cleaning, compatibitity with other ingredients and their physical properties. An important property of a silica for use in transparent toothpaste formulations is its apparent refractive index, and the lower this value in the chosen water/humectant system the more water can be allowed for by the formulator in the transparent dentifrice. This replacement of the more expensive humectant e.g. Sorbitol and/or Glycerol, by water leads to substantial economic benefits to the formulator.
The inventors have shown that the apparent refractive index of an amorphous silica can be controlled by the careful selection of process conditions at the precipitation stage of the preparation of the silica. It can be demonstrated that changing such conditions as pH, electrolyte level and silica concentration alters the fundamental features of the overall pore size distribution present in the amorphous silica. Amorphous silicas can be prepared with an extremely broad pore size distribution stretching from ultramicropores (below 0.7 nm), through supermicropores (0.7 to 2 nm), continuing to mesopores (2 nm to 50 nm), finally macropores (above 50 nm), in accordance with the classification given in, Characterisation of Porous Solids; An Introductory Survey by K. W. S. Sing in Characterisation of Porous Solids II 1991 Elsevier Science Publishers BV Amsterdam.
It is postulated that the level of ultramicropores, which are defined as pores small enough to exclude the nitrogen molecule, controls the degree of movement in the apparent refractive index of the amorphous silica when it is brought into contact with the water/humectant system. As the number of ultramicropores increases, there will be a higher propensity to preferentially adsorb water from the water/humectant system and hence lower the apparent refractive index of the silica.
Surprisingly, silicas of the invention retain their ultramicropore distribution when subjected to ageing and therefore retain their apparent refractive index position. Additionally, the silicas retain good clarity making them suitable for use in transparent toothpaste formulations.
The inventors have shown, by careful selection of process conditions, followed by controlled subsequent ageing, amorphous silicas can be prepared with extremely low surface areas (below 100 m.sup.2 g.sup.-1), having a low apparent refractive index of approximately 1.44, but maintaining medium to high abrasivity and excellent clarity when the silica is dispersed in a water/humectant system.